1. Field of the Invention
This invention relates to plasticizers for organic polymers. More particularly, this invention relates to benzoate monoesters that are useful as plasticizers for solventless and non-aqueous plastisols containing organic polymers.
2. Description of the Prior Art
Using combinations of mono and diesters derived from benzoic acid and dihydric alcohols as plasticizers for aqueous plastisols containing organic polymers is known.
The use of diesters derived from benzoic acid and a dihydric alcohol such as ethylene glycol in combination with limited amounts of the corresponding monoesters as plasticizers for aqueous polymer emulsions, one type of plastisol, is described in the prior art. As an example of the latter, U.S. Pat. No. 5,676,742, which issued to William Arendt on Oct. 14, 1997 describes aqueous plastisols useful as latex caulks. The primary plasticizer in these compositions is a diester derived from benzoic acid and diethylene and/or dipropylene glycol. Both hydroxyl groups of the diol are esterified.
In accordance with the teaching of the aforementioned patent to Arendt, a shortcoming of caulks formed from aqueous plastisols containing dibenzoates of diethylene- and/or dipropylene glycol as plasticizers is the greater susceptibility of the final caulk to fungal attack relative to caulks prepared using the same film-forming polymer(s) and a diester of phthalic acid such as butylbenzyl phthalate as the plasticizer.
The Arendt patent teaches that even though monoesters of glycols are not considered effective plasticizers for certain applications, relatively low levels of diethylene glycol (DEG) monobenzoate and/or dipropylene glycol (DPG) monobenzoate in combination with the corresponding dibenzoate as the primary plasticizer were sufficient to improve the fungal resistance exhibited by the caulks relative to the resistance imparted by dibenzoate plasticizers without any substantial adverse effect on the processability of the plastisol.
Throughout this patent the concentrations of the glycol or diol monobenzoates in blends containing the corresponding dibenzoates are expressed in terms of a “hydroxyl number”. The units for the “hydroxyl number” are understood by those skilled in the art to be milligrams of potassium hydroxide per gram of sample.
The hydroxyl numbers exhibited by the mixtures of diethylene glycol mono- and dibenzoates disclosed in the Arendt patent range from 12.1 to 58.4. This is equivalent to a concentration of diethylene glycol monobenzoate in the mixture of from 4 to 18 weight percent.
The use of the monobenzoate of 2,2,4-trimethyl-1,4-pentanediol as a plasticizer for polyvinyl chloride is described in U.S. Pat. No. 5,006,585, which issued to DiBella on Apr. 9, 1991. Because polyvinyl chloride is processed as a molten material, any adverse effect of the plasticizer on the viscosity or processability of the plastisol is not a consideration. The only criterion for the melting point of the plasticizer is that it be below the melting point of polyvinyl chloride.
Monoesters derived from benzoic acid and dihydric alcohols are highly polar compounds due to the presence of a hydroxyl group on the alcohol portion of the ester. This polarity is believed responsible for the observed limited compatibility of the monoesters with less polar polymers such as polyvinyl chloride and other halogen-containing polymers.
Diesters derived from benzoic acid or other aromatic monocarboxylic acid and dihydric alcohols such as diethylene and dipropylene glycols are very compatible with polyvinyl chloride. This high level of compatibility may increase the viscosity of a plastisol containing finely divided polymer particles and an effective concentration of the plasticizer to the extent that the composition cannot be readily processed using conventional equipment.
One approach to reducing the viscosity of plastisols containing diesters of aromatic monocarboxylic acids as the primary plasticizer has been the use of esters derived from monohydric alcohols and aliphatic monocarboxylic acids as secondary or auxiliary plasticizers. Secondary plasticizers exhibit only limited compatibility with the polymer and would be expected to reduce the viscosity of the plastisol.
The present invention is based on the discovery that monoesters derived from a dihydric alcohol and a monocarboxylic acid such as benzoic acid can be used as secondary plasticizers for non-aqueous and solventless plastisols containing primary plasticizers that include but are not limited to the aforementioned aromatic acid diesters.
An additional feature of these monoesters resides in their ability to reduce the viscosity of a plastisol while simultaneously reducing the temperature required for fusion of the dispersed polymer particles into a unitary article and remaining compatible with the polymer.
Use of the present secondary plasticizers may also impart additional advantages including but not limited to reduced cost and increased stain resistance of the final polymer composition.
The secondary plasticizers of this invention are also suitable for use with polymers in the form of solid particles that remain in this form following treatment with a plasticizer.